Depends a lot on your personal situation, and jurisdiction.

• Doing a ceremony when you publicly say you love each other is already a valid reason

• In some jurisdiction, you’d get a form of tax benefit for being married, it often comes with downside like having welfare benefit based on the couple revenue rather than on individual ones (hence the tax benefit). Talk with an accountant/Tax-lawyer knowing your local laws for details

• It gives a legal status to your shared asset. Sure you could create a real-estate-investment company to buy your house and many people do that but being married, with a proper prenup give you a lot of agency regarding your shared asset

• It protects the weaker partner, usually the one scarifying their carrer for the couple if things goes wrong